One of factors for letting an automobile safely travel includes pressure of tires. When the pressure is decreased to less than a proper value, driving stability and fuel consumption are deteriorated, so that a tire burst may sometimes be caused. Therefore, a tire pressure monitoring system (TPMS) for detecting a decrease in tire pressure and issuing a warning so as to urge a driver to take a proper action is an important technique from a view of protecting an environment and ensuring safety of the driver.
The conventional monitoring system can be classified into two types including a direct detection type and an indirect detection type. The direct detection type is to directly measure the pressure of the tire by an incorporating pressure sensor inside a tire wheel. Although the decrease in the pressure can be detected with high precision, some disadvantages in terms of technique and cost remain including a need for a dedicated wheel and a problem in fault tolerance in an actual environment.
Meanwhile, the indirect detection type is a method for estimating a deflated state from rotation information of the tire, and can further be classified into a DLR (Dynamic Loaded Radius) method and a RFM (Resonance Frequency Method) method. The DLR method is a method for utilizing a phenomenon that a dynamic loaded radius is decreased due to crush of a deflated tire during traveling and as a result the tire is rotated more quickly than a tire of normal pressure, and comparing rotation speed of four tires so as to detect the decrease in the pressure (for example, refer to Patent Literature 1). Since calculation processing is relatively easily performed with using only rotation speed signals of wheels obtained from wheel speed sensors, the DLR method has been widely studied for a purpose of mainly detecting a puncture of one wheel. On the other hand, the RFM method is a method for utilizing a change in a frequency characteristic of wheel speed signals due to deflation so as to detect a difference from normal pressure (for example, refer to Patent Literature 2). Due to absolute comparison with preliminarily held normal values of wheels, the RFM method is drawing attention as an indirect detection method capable of detecting simultaneous deflation of four wheels unlike the DLR method.